1. Field of the Invention
The present invention relates generally to a method and a system for controlling a vertically movable fork supported by the upright of a fork lift truck at a position parallel with respect to the ground, by the aid of a microcomputer, and more specifically to a method and a system for adjusting the backward tilting angle of the upright (masts) so that the fork is always positioned at a position parallel with respect to the ground even when the load mounted on the fork changes.
2. Description of the Prior Art
An automatic lift operation of a fork lift truck has been proposed by using a microcomputer. In this case, the automatic lift operation is carried out by outputting a number of operation command signals from the microcomputer on the basis of comparison of the current fork lifting height detected by a lift sensor with the reference fork lifting height previously stored in a microcomputer.
When a fork lift truck is used for lifting a load, the lifting height is important. This lifting height is usually obtained by detecting the travel distance of a chain or the number of revolutions of a chain wheel.
In this case, if the fork is accurately positioned at a position parallel with respect to the ground, the lifting height between the fork horizontal portion and the ground is constant everywhere throughout the fork horizontal portion; however, if the upright tilts forwardly or backwardly and therefore the fork is not positioned parallel with respect to the ground, the lifting height at the free end of the fork horizontal portion is not equal to that at the fixed end of the horizontal portion of the fork which is calculated on the basis of travel distance of the chain.
To overcome this problem, therefore, in the usual automatic lifting operation of the fork lift truck, the upright by which the fork is supported movably up and down is first adjusted vertically to the ground. However, in the case where some load is applied onto the fork, there exists a problem in that the fork is tilted forwardly and, additionally, the forward tilting angle varies according to the degree of load. This is because the load on the fork will deform the front wheels, the mast, the fork horizontal portion, etc.
Furthermore, in the usual automatic upright vertically controlling operation of the fork lift truck, since the upright is usually stopped abruptly at its vertical position with respect to the ground, there further exists another problem in that a shock is applied to the load mounted on the fork and thereby there is created a danger that the load may be moved or thrown off the fork, whenever the upright is stopped abruptly.
Furthermore, if an electric failure occurs in the analog-to-digital converter or in a potentiometer for detecting the fork tilt angles, a bit error is produced within any of the output bit strings from the analog-to-digital converter to be fed into the microcomputer. Consequently, since the microcomputer performs the feedback control for the tilt cylinder on the basis of the erroneous bit strings, the upright is tilted at an erroneous angle different from the target value described above so that, e.g., the loads are not properly arranged on the fork and accordingly an accident may occur due to improper arrangement of the loads.
The more detailed description of the prior-art fork lift truck will be made hereinafter with reference to the attached drawings under DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS.